A system for wide-range spectral measurement includes one or more broadband sources, an adjustable Fabry-Perot etalon, and a detector. The one or more broadband sources is to illuminate a sample, wherein the one or more broadband sources have a short broadband source coherence length. The adjustable Fabry-Perot etalon is to optically process the reflected light to extract spectral information with fine spectral resolution. The detector is to detect reflected light from the sample, wherein the reflected light is comprised of multiple narrow-band subsets of the illumination light having long coherence lengths and is optically processed using a plurality of settings for the adjustable Fabry-Perot etalon, and wherein the plurality of settings includes a separation of the Fabry-Perot etalon plates that is greater than the broadband source coherence length but that is less than the long coherence lengths.